1. Field of the Invention
The present invention relates to multi-color projection systems, and more particularly concerns methods and apparatus for automatic correction of misconvergence of plural images of a multi-image projection system.
2. Description of Related Art
A common type of color projection system employs three separate but closely positioned cathode ray tube projection devices, each provided with a separate projection lens system that directs the image of each cathode ray tube to a common region of a projection screen so that three separate monochromatic color images are superposed on one another to provide a single multi-color image. Precision of superposition of the three different color images is essential in such a system to avoid degraded resolution and rough, blurred composite images. To enhance precision of convergence of all three separately colored monochromatic images, various types of convergence systems have been developed. Manual systems are labor intensive, tedious and extremely time consuming, requiring time measured in the order of hours, for example. In addition, the manipulator must have technical knowledge or training sufficient to execute the correction. Such ability is seldom available to the average video projector purchaser. Automatic convergence systems avoid disadvantages of manual systems but are subject to limitations with respect to accuracy, speed and reliability. For example, U.S. Pat. No. 4,999,703 for Automatic Image Correction Method and Apparatus For Projectors Utilizing Cathode Ray Tubes provides for automatic convergence by use of a mechanically scanning optical head that samples certain predetermined areas of a projected test pattern. The head is mechanically moved to scan areas of the projection screen in sequence, and of course requires various mechanical scanning elements and motors. Mechanical elements detract from system reliability. The requirement for servo motors greatly adds to cost and complexity of the system. Moreover, because of the sequential physical scanning of such systems, achieving convergence still requires several minutes of time. The system of U.S. Pat. No. 4,999,703 thus involves decreased reliability and increased cost while accuracy of convergence is dependent upon accuracy limits of the motion of the motors.
Another similar type of convergence system employing a lens and sensor that are moved as a unit to point to different areas of the screen by a pair of stepper motors is described in an article entitled "A Versatile Automatic Convergence System for a Three-Lens CRT Projector", by AK Oebel and T. Schmidt, SID 91 Digest pages 159-169.
Accordingly, it is an object of the present invention to provide for automatic convergence by use of methods and apparatus that avoid or minimize the above mentioned problems.